As shown in FIGS. 11 and 12, a conventional multi-blade centrifugal fan is formed of a fan casing 1 and a multi-blade centrifugal impeller 2. The fan casing 1 is provided with bellmouths 4 each forming an air intake. A number of blades 6 are annularly arranged in the impeller 2, which blows out air W drawn in through the intakes 7 which face the above described bellmouths 4 in the centrifugal direction through the above described blades 6. The outer peripheral end portions of the above described impeller 2 are provided with retainer rings 10 for retaining the above described blades 6 (see Patent Document 1). The impeller 2 is provided with a main plate 8 and a bearing 9.    Patent Document 1: Japanese Laid-Open Patent Publication 2001-173596